1. Field of the Invention
The present invention relates to an Fe-Ni alloy sheet, which is excellent in hot workability, adhesivity to a plating layer, and solderability, and is particularly applicable as a material for a lead frame of an integrated circuit, and a method for manufacturing same.
2. Related Art Statement
A conventional Fe-Ni alloy widely used as a material for a lead frame of an integrated circuit consists essentially of 42 wt. % nickel and the balance of iron and incidental impurities with a view to minimizing the difference in the thermal expansion coefficient from the other component materials of the integrated circuit, i.e., a semiconductor, glass and ceramics.
A lead frame of an integrated circuit comprising such an Fe-Ni alloy is usually manufactured as follows: Preparing an ingot or a continuously cast slab of the above-mentioned Fe-Ni alloy; subjecting the ingot or the continuously cast slab thus prepared to a slabbing rolling, a hot rolling and a cold rolling to prepare an Fe-Ni alloy sheet; slitting the thus prepared Fe-Ni alloy sheet into a plurality of belts having a prescribed width; and subjecting the thus obtained belt to a punching or a photoetching to manufacture a lead frame having a prescribed shape. A noble metal plating such as a silver plating is usually applied to the body portion of the thus manufactured lead frame except for the legs thereof to form a noble metal plating layer such as a silver plating layer on the surface of the body of the lead frame, thereby improving operability of a wire-bonding.
Then, an integrated circuit is manufactured by subjecting the lead frame to a die-bonding, a wire-bonding and a packaging in this order.
The legs of the thus manufactured integrated circuit are soldered to the substrate sheet. A tin plating is applied to the legs to improve solderability thereof.
As is clear from the above description, the Fe-Ni alloy sheet used as a material for a lead frame is required to be excellent both in adhesivity to a plating layer, particularly to a silver or gold plating layer and in solderability.
However, the above-mentioned conventional Fe-Ni alloy sheet has a poor adhesivity to a silver plating layer. More particularly, in an integrated circuit manufactured by the use of a lead frame made from the conventional Fe-Ni alloy sheet, heat produced when subjecting the lead frame to a wire-bonding may cause the production of "blisters" in the silver plating layer or the peel-off of the silver plating layer. In order to solve such problems, therefore, it is necessary, as the conventional practice, to apply a strike plating to the body of the lead frame with nickel or copper prior to the silver plating of the body of the lead frame, resulting in operational complication and increase in the manufacturing cost.
In the integrated circuit manufactured using a lead frame made from the conventional Fe-Ni alloy sheet, furthermore, when tin-plating the legs of the integrated circuit for the purpose of improving solderability, as described above, fine acicular crystals (hereinafter referred to as the "whiskers") tend to be produced in an abnormally large amount in the tin plating layer. This abnormal production of the whiskers leads to a longer wetting time between the tin plating layer and the solder, thus resulting in more difficult adhesion of the solder to the legs of the lead frame applied with the tin plating, and hence in a lower solderability.
For the purpose of solving the above-mentioned problem, Japanese Patent Provisional Publication No. 62-207,845 dated Sep. 12, 1987 discloses an Fe-Ni alloy sheet excellent in adhesivity to a plating layer, which consists essentially of:
______________________________________ nickel: from 35 to 55 wt. %, aluminum: from 0.010 to 0.100 wt. %, calcium: from 0.0005 to 0.0040 wt. %, carbon: up to 0.05 wt. %, nitrogen: up to 0.005 wt. %, sulfur: up to 0.003 wt. %, chromium: up to 0.5 wt. %, and the balance being iron and incidental impurities. ______________________________________ (hereinafter referred to as the "prior art").
The object of the prior art is to improve adhesivity to a silver plating layer without applying a nickel strike plating to the lead frame made from the Fe-Ni alloy sheet of the prior art. In order to achieve this object, aluminum and calcium in respective prescribed amounts are added to the Fe-Ni alloy in the prior art.
In order to improve adhesivity to the silver plating layer in the prior art, however, the silver plating layer must have a thickness of at least 3 .mu.m. The silver plating layer, having such a large thickness, results in a higher manufacturing cost of the integrated circuit. With a view to reducing the manufacturing cost of the integrated circuit, therefore, the silver plating layer formed on the surface of the lead frame made from the Fe-Ni alloy sheet should preferably have a thickness of under 3 .mu.m. However, even when the silver plating layer has such a thin thickness, it is impossible by the prior art to ensure an excellent adhesivity to the silver plating layer.
In addition, in the integrated circuit manufactured using the lead frame made from the Fe-Ni alloy sheet of the prior art, application of the tin plating to the legs of the integrated circuit with a view to improving solderability, causes the production of whiskers in an abnormally large amount in the tin plating layer, thus resulting in considerable decrease in solderability. Such an abnormal production of whiskers is attributable to the fact that an aluminum oxide film having a non-uniform surface is formed on the surface of the lead frame under the effect of the annealing during the manufacturing process of the lead frame because of the aluminum content of at least 0.010 wt. in the Fe-Ni alloy sheet of the prior art.
Furthermore, an ingot or a continuously cast slab of the Fe-Ni alloy of the prior art is poor in hot workability, and the slabbing rolling of such an ingot or a continuously cast slab causes the production of considerable flaws on the surface of the resultant finished slab, thus resulting in a lower production yield.
Under such circumstances, there is a strong demand for the development of an Fe-Ni alloy sheet, which is excellent in hot workability, adhesivity to a plating layer and solderability, and is high in the production yield, and a method for manufacturing same, but such an Fe-Ni alloy sheet and a method for manufacturing same have not as yet been proposed.